fix: recent build warnings and errors (#5711)

* fix: use std::empty instead of isEmpty()

* fix: fix trivially assignable warning hicpp-move-const-arg

* fix: fix clang-diagnostic-unused-but-set-variable warning

* fix: fix readability-braces-around-statements warning

qt/RpcQueue.cc

@@ -48,7 +48,7 @@ void RpcQueue::stepFinished()
     else
     {
         assert(!next_error_handler_);
-        assert(queue_.isEmpty());
+        assert(std::empty(queue_));
 
         // one way or another, the last step returned nothing.
         // assume it is OK and ensure that we're not going to give an empty response object to any of the next steps.
@@ -61,7 +61,7 @@ void RpcQueue::stepFinished()
 
 void RpcQueue::runNext(RpcResponseFuture const& response)
 {
-    assert(!queue_.isEmpty());
+    assert(!std::empty(queue_));
 
     auto next = std::move(queue_.front());
     queue_.pop();